Ion implanters are commonly used in the production of integrated circuits (IC) to create in a semiconductor wafer, usually silicon, regions of different conductivity by p- or n-type doping. In such devices, a plasma source is used to ionize the dopant gas. A beam of positive ions is extracted from the source, accelerated to the desired energy, mass filtered and then directed toward the wafer. As the ions strike the wafer, they penetrate to a certain depth (depending on their kinetic energy and mass) and create regions of different electrical conductivity (depending on the dopant element concentration) into the wafer. The n- or p-doping nature of these regions, along with their geometrical configuration on the wafer, define their functionality, e.g., n-p-n or p-n-p junctions within the transistors. Through interconnection of many such doped regions, the wafers can be transformed into complex integrated circuits.
A block diagram of a representative ion implanter 50 is shown in FIG. 1. Power supply 1 delivers the required energy (DC or RF) to the plasma source 2 to enable ionization of the doping gas. The gas is fed into the plasma chamber through a mass-flow controlled system (not shown) under the pressure in the mTorr range, ensured by a vacuum pumping system. Depending on the desired dopant, different fluoride or hydride doping gases, such BF3, PF3, AsF3, GeF4, B2H6, PH3, AsH3, GeH4 or others, with or without co-carrier gas, are introduced. The plasma chamber has an aperture 3 through which the ions are extracted by a combination of electrodes 4. A commonly used scheme is a triode combination in which the plasma chamber aperture is at high positive potential, then a second electrode (suppression electrode) at negative potential and finally a third electrode at ground potential. The role of second electrode is to prevent secondary electrons from streaming back to the plasma chamber. However, other extraction electrode combinations such as thetrode, pentode or Einzel lenses are also possible. These exiting ions are formed into a beam 20, which then passes through a mass analyzer magnet 6. The extracted ion beam is composed of a mixture of ions. For instance, the beam extracted from BF3 gas will be comprised mainly of BF3+, BF2+, BF+, B+, and F+ ions. Therefore, it is necessary to use the mass analyzer to remove unwanted components from the ion beam, resulting in an ion beam having the desired energy and composed of a single ionic specie (in the case of BF3, the B+ ion). To reduce the energy to the desired level, ions of the desired species then pass through a deceleration stage 8, which may include one or more electrodes. The output of the deceleration stage is a diverging ion beam. A corrector magnet 10 is used to expand the ion beam and then transform it into a parallel ribbon ion beam. Following the angle corrector 10, the ribbon beam is targeted toward the wafer or workpiece. In some embodiments, a second deceleration stage 12 may be added. The wafer or workpiece is attached to a wafer support 14. The wafer support 14 provides a vertical motion so that the wafer can be brought in the beam path and then passed up and down through the fixed ion ribbon beam. It also can be rotated so that implants can be performed at different incidence angles with respect the wafer surface. With the wafer out of the beam path, the beam current can be measured by a Faraday cup 16. Based on the beam current value and the desired dose, the wafer exposure time or the scanning speed and the number of passes through the ribbon ion beam is calculated.
Taking into account that the rate of ion extraction from the plasma source is given bydNextr/dt≅AnvB where A is the area of the extraction aperture, n the ion density (supposedly equal to electron density), and vB=(kBTe/mi)1/2 the Bohm velocity (with kB, Te and mi the Boltzmann constant, electron temperature and ion mass, respectively) a limited number of plasma sources have proved to have sufficient plasma density to be useful as ion sources. In some embodiments, such as Bernas sources, an arc discharge creates the plasma. Tungsten filaments are used to generate a flux of electrons needed to sustain the high arc plasma density. In other embodiments, such as indirectly heated cathodes (IHC) which are also a form of arc discharge, to prevent the filament from detrimental exposure to the plasma and therefore to extend the lifetime of the source, the necessary electrons are provided by thermionic emission from an indirectly heated cathode. While these quasithermal plasma sources are effective in generating the desired ion densities, they are typically only used to create atomic ions, due to the high temperatures developed within the arc chamber. Because dissociation energies are typically low, the thermal energy in the arc plasma is often high enough to breakdown molecular bonds and to fractionate the feeding gas into smaller molecules or atoms.
It has been found that for shallow implants applications where low ion energy is required, in order to overcome the detrimental space-charge effects and to increase the productivity of the ion implantation process, molecular gases with higher content of the active dopant in the molecule such as C2B10H12, B10H14, and B18H22 can be used. The resulting molecular ions can be accelerated at higher energies, thus preventing the beam from the space-charge detrimental effects. However, due to their heavier mass, shallow implants can be performed.
For such implantation processes that require molecular ions rich in active dopant rather than dopant atomic ions, low temperature plasma sources such as RF inductively coupled discharges are well suited. In these discharges, the plasma is produced by coupling the power from an RF generator through an antenna. One such source is an inductively coupled plasma source (ICP). The high RF currents flowing through the antenna give rise to an oscillatory magnetic field which, according to the Maxwell's 3rd electrodynamics law:∇×{right arrow over (E)}=−∂{right arrow over (B)}/∂t produces intense electric fields in a limited spatial region (skin depth) which is a function of the RF excitation frequency and gas pressure. Electrons accelerated by these electric fields gain enough energy to ionize the gas molecules and create a plasma. The created plasma is not in thermal equilibrium since electrons have a temperature (usually ˜2-7 eV) much higher than ion or neutral temperature.
Another potential plasma source for ion implantation purposes is an electron cyclotron resonance (ECR) source. The working principle of ECR source utilizes the electron cyclotron resonance to heat the plasma. Microwaves are injected into a volume, at the frequency corresponding to the electron cyclotron resonance as defined below. The volume may contain a low pressure gas. The microwaves may heat free electrons in the gas which in turn collide with the atoms or molecules of the gas in the volume and cause ionization.
In a cold plasma, a wave propagating along the magnetic field obeys the following dispersion relation
  N  =            1      ±                                    (                                          f                pe                            f                        )                    2                ⁢                  1                                                                      f                  ce                                f                            ⁢                                                k                                                  k                                      ∓            1                              where N is the refraction index, fpe=(nee2/4π2ε0me)1/2 is the plasma frequency (with ne, e, ε0, and me the electron density, elementary charge, dielectric constant of the vacuum, and electron mass, respectively), fce=eB/2πme is the electron cyclotron frequency (B is the induction of the magnetic field), k and k∥ are the total and parallel with the magnetic field wave numbers. The equation that implies “+” sign before the fraction corresponds to the right hand polarized wave and the other (“−” sign) to the left hand polarized wave. Relevant to ECR sources are the right hand polarized (RHP) waves because they may propagate for arbitrarily high plasma densities for magnetic field strengths for which the cutoff is absent. More important, RHP waves have a resonance at the electron cyclotron frequency which means the plasma can efficiently be heated by coupling the power to the electronic component. For the most common microwave frequency (2.45 GHz), the resonance condition is met when the magnetic field strength is B=875 Gauss.
Due to its simple design (helical antennae for ICP sources, ring magnets for ECR sources) cylindrical geometry was adopted for such plasma sources. The drawback for this geometry is that the plasma is radially non-uniform, i.e., the plasma column has a very peaked density profile on the axis of the discharge. This non-uniform plasma density profile along radial direction characteristic limits the application of this geometry for large area plasma processing. Therefore, typically a processing (diffusion or expansion) chamber may be used in conjunction with the source so that the plasma generated in the plasma source expands within the processing chamber and the peaked density profile relaxes. However, although smoother, for some applications, the density profile is unacceptable because it still tracks the plasma density profile in the source, as seen in FIG. 2.
Therefore, an ion source that can effectively utilize the relatively high plasma density produced by the ICP and/or ECR plasma sources and create a wide and uniform ribbon ion beam would be beneficial from ion implantation perspective.